Why Did T Rex Have Such Tiny Arms?


Hi, this is Kate from MinuteEarth. Paleontologists have long questioned why Tyrannosaurus
Rex – a giant killing machine with 9-inch-long daggers for teeth – had such puny arms. Were short arms particularly useful when grappling
with prey, or when Sexy Rexy was in mating mode? Did they provide just the right leverage when
getting up after a nap? Maybe. Like some evolutionary traits, from the gecko’s
camouflage, to the human’s opposable thumb, to the orchid’s deceiving shape, perhaps those
little limbs gave T. Rex some real advantage. But it’s also quite possible that they didn’t. For one thing, T. Rex might have ended up
with small arms simply because big – or even normal-sized – arms were a DISadvantage. Perhaps once T-rex’s ancestors got big enough,
they could hunt and eat with their giant jaws alone – and their arms weren’t worth the
energy they took to haul around. But evolution can’t just swap out not-so-useful
traits for completely new ones. Instead, over generations, T. rex’s arms
may have simply gotten smaller and smaller until there was no longer a significant cost
to keeping them. Then they just…stuck around – a lot like
how we humans ended up with our tiny tailbones. Another possibility is that evolution gave
T. Rex tiny arms simply by chance, the same way it gave Ireland lots and lots of redheads. In a relatively small, isolated population,
a trait that doesn’t hurt OR help an individual’s chances of surviving and reproducing can,
just by the luck of the draw, become more and more prevalent from one generation to
the next. It’s possible that the size of T-rex’s
arms didn’t matter much either, and all the dinos just happened to end up with short
arms. On the other hand, weirdly-small arms have
also evolved in other large T-rex relatives, suggesting that the trait might not be totally
random. Right now, scientists are studying the microscopic
wear and tear on the arm bones of the most famous T-rex of all to determine how she used
them. This work might help us figure out if T. rex’s
arms were useful like our thumbs, useless like our tailbones, or random like our redheadedness. Right now, the answer is still out of reach. This video was sponsored by the University
of Minnesota, where students, faculty and staff across all fields of study are working
to solve the Grand Challenges facing society. University of Minnesota professors Suzanne
McGaugh and Emma Goldberg, who advised us on the science behind this video, investigate
how several different mechanisms of evolution, from adaptation to negative selection to genetic
drift, affect traits — like T. rex’s tiny arms — in a variety of species. Professor Goldberg uses math to explore how
a plant’s reproductive traits affect its chances at becoming new species or going extinct,
and professor McGaugh rappels into Mexican caves to better understand how blindness in
cavefishes evolves independently in different populations. Thanks, University of Minnesota!

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